Over the last 75 years, hot rodders have tried souping-up their engines with every possible exotic fuel and power additive. After trial and error, they settled on nitromethane-a dry-cleaning solvent and sometime rocket fuel-as by far the most potent. High loads of nitro are so volatile and add so much power that engines run with it are literally skating on the edge of destruction. Nitro's volatility, its unique cackling sound, the flames shooting out the exhaust pipes-it's all become the source of myth and legend. To get the straight scoop on using nitro, we consulted a number of experts, including nitro pioneer Gene Adams, who still builds fuel Hemis; Jim Archer, who has mixed just about every possible fuel with every other fuel; Jeff Prock, purveyor of high-end nitrous oxide systems (yes, you can mix nitro and NOS); and the very informative data at Ray Hall Turbo's Australian-based website (TurboFast.com.au). We think you'll find that nitro truth is a lot stranger then any fictional tall tales.

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What is Nitromethane, Anyway?

Nitromethane-or CH3NO2-is one member of a family of explosive compounds that contain nitrogen and oxygen. Remember the old safecracker crime movies where the "specialist" had to be real careful with the small vial of nitroglycerin, lest any sudden impact cause it to explode? Then there's TNT (trinitrotoluene) and gunpowder (nitrocellulose). Yup, anything with "nitro" in it is bad stuff!

What makes them so bad is the oxygen in the nitro group, which breaks down into gaseous combustion products that create large amounts of heat and pressure without the need for further oxygen. Nitromethane and its relatives have the potential to be monopropellants; they can combust without any air at all. That's why nitromethane was once used as a rocket fuel. Fortunately for hot rodding, nitromethane also has industrial-world uses-for example, as a cleaning solvent and as an aid for synthesizing pharmaceuticals, pesticides, and coatings-which is why it's widely available.

Who's On First?

Reputedly the first to use nitro as a fuel were model tether-car racers; the current descendents exceed 200 mph, circling a pole on a string. For fullsize race cars, the first documented use was in the mid-to-late-'30s Auto Union Grand Prix and land speed record cars designed by Ferdinand Porsche. These were subsidized by the Nazis, who wanted to prove the superiority of German technology. The open-wheel racers had streamlined bodies and were decades ahead of their time, but most of the technology was lost when the Third Reich collapsed at the end of World War II. The last version built before the war was a 340ci (5,577cc) V12 with nonintercooled, two-stage Roots superchargers rated at 485 hp and 405 lb-ft. The fuel is believed to have been a mix of 85 percent nitromethane, 5 percent benzole, 5 percent acetone, and 5 percent castor oil, with fuel consumption around 2.5 miles per gallon. German racing driver Berndt Rosemeyer was killed in one of these cars when running 268 mph on the Autobahn in attempt to set a new world land speed record.

Totally unaware of the Nazi efforts, American hot rodders reinvented the technology in the late '40s. According to most hot rod historians, nitro's first competition use in America was by Vic Edelbrock Sr. and his associates. As related by Vic Edelbrock Jr., who was there when the events went down, back in '49, Midget racer Ed Haddad came into the shop after he'd been given 1 gallon of a nitromethane-based fuel by slot-car manufacturer Dooling Brothers, but didn't want any part of it because he had heard "it will blow up in your face." Edelbrock, Bobby Meeks, and Fran Hernandez added 10 percent nitro to the methanol in their 136ci V8-60 Midget car engine. With no tuning or familiarity with nitro, Vic Jr. recalls that the strange brew "just about broke the beam on Dad's old 200hp-capacity Clayton dyno. The spark plugs were so hot they turned into glow plugs. When they tried to shut it off, the engine kept running. They finally had to throw a towel on it to get it to quit." The engine was toast, but eventually they learned to add lots more fuel, colder spark plugs, and stronger internals to stand up to both the higher output as well as nitromethane's corrosive effects. The Stromberg 81 carbs had to be nickel-plated, as did the fuel containers (hidden from prying eyes inside cardboard boxes). Eventually, Edelbrock settled on a 20 percent nitro/80 percent methanol mix that added 40 hp.

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Later in 1949, powered by the secret sauce, Edelbrock's Fran Hernandez's '32 Ford beat Tom Cobbs' blown Roadster at the Goleta, California, airport in the first sanctioned drag race ever held. But what really made everyone take notice was Vic Edelbrock Sr.'s circle-track Midget driven at the famous Gilmore Stadium in 1950 by future Indy 500 winner Roger Ward. At the time, Midget racing was the rage in Southern California. Purpose-built Offenhauser four-cylinder racing engines had a significant power advantage over other setups and dominated the top circuit. The Ford V8-60 flatheads were relegated to their own class. But on that historic night at Gilmore, Edelbrock entered its Kurtis Kraft V8-60-powered Midget in the Offenhauser class, fueled by his secret brew, and smoked them all. It was the only time a Ford V8-60 won at Gilmore over a field of Offys. In San Bernardino the following night, Edelbrock blew off the Offys again.

Edelbrock was able to keep the fuel a secret for a while, but with flames coming out of the exhaust, fellow racers knew something was up. Vic disguised the distinctive odor by blending in a little orange oil. By 1952, an Edelbrock Ford flathead running 40 percent nitro had run 201 mph one way at Bonneville (before the exhaust valves got sucked into the ports). Other racers experimented with fuel in the early '50s, including Joaquin Arnett of Bean Bandits fame. Tony Capana is said to have been the first to take nitromethane to the dry lakes, and, by 1954, to the Indy 500 (where it was legal at the time).

The Rich Get Richer

According to Gene Adams, if you consider high-octane racing gasoline as the baseline fuel, replacing it with methanol-the best alcohol fuel-is worth a 5-to-10-percent power gain. But 80-to-90-percent nitro is worth two to three times the power of the alky.

What's the secret? Nitromethane carries its own oxygen, so it needs much less atmospheric oxygen to burn. The theoretical ideal or stoichiometric air/fuel ratio for gasoline is 14.7:1. That means, 14.7 pounds of air are needed to burn 1 pound of gas. Methanol, which also carries oxygen, has a stoichiometric ratio of 6.45:1. But with 100 percent nitro, the ratio is 1.7:1! Because the displacement of an engine cylinder is fixed, this means-assuming 100 percent volumetric efficiency (VE)-8.7 times more nitromethane than gasoline can be burned during one combustion cycle.

On paper, gasoline has about four times more heating value than nitromethane: at least 19,000 Btu/lb for gas compared with just 4,850 Btu/lb for nitro. But that doesn't take into account the fuel's specific energy (SE) value, which is derived by dividing the heat value by the air/fuel ratio (Btu/lb ÷ A/F), telling us how much heat energy is delivered per pound of air into the motor. At stoichiometric air/fuel ratios, the nitro's SE value is around 2.2 times greater than gasoline!

Racing nitro motors run far richer than the theoretical 1.7:1 ratio, and Adams says it's possible to dump nitro at ratios approaching 0.5:1. "At 80 percent nitro and above, the optimum air fuel/ratio no longer exists and the power output becomes well related to the actual amount of fuel fed into the engine by weight," adds Ray Hall Turbo. At 0.5:1, the SE potential of nitro could be six times greater than gas.

"Gas is for washin' parts. Alcohol is for drinking. Nitro is for racin'!" -Anonymous Racer

Compared to methanol, nitro's theoretical SE advantage is nearly 40 percent at stoichiometric and more than 110 percent at theoretical max power ratios. When you add in nitro's high heat of vaporization (about twice that of methanol), you also get a significant cooling effect in the chamber. Since nitro wants to explode instead of burn in a controlled manner like a properly tuned gasoline-fueled engine, anything you do to reduce chamber hot spots is critical!

All this still doesn't take into account that at extremely rich ratios, the nature of nitro's chemical reaction under combustion changes, producing new end products including hydrogen-another compound that really likes to go "boom" (remember the Hindenburg?).

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The 98 percent Solution

Although it's possible to run 100 percent nitro-Art Chrisman is said to have done so, with carburetors to boot-experts like Gene Adams don't recommend it. "Even if the rules allow it," Adams says, "cutting nitro with another fuel makes the car more consistent. It'll run cleaner and there's less tendency to drop cylinders. In my experience 98 percent is best overall."

Thanks to improvements in magneto technology, 98 percent is doable today. The ignitions of the past just weren't up to the job. Regardless, NHRA currently restricts Top Fuel and Funny Cars to a 90 percent nitro solution in an effort to hold down speeds. A normally-aspirated A/Fuel dragster can run 94 percent nitro. At Bonneville, it's still "run what ya brung."

Methanol Brews

Methanol remains the most popular fuel used to cut nitromethane, if only because many sanctioning bodies currently ban the alternatives. Nevertheless, there's a good reason to cut nitro with up to 10 percent methanol: It helps suppress detonation. Ray Hill Turbo recommends a 2.5 percent water/7.5 percent methanol cut to reduce both preignition and detonation tendencies with, it claims, "almost the full power capability of undiluted nitromethane."

Weird Science

When running nitro absent of rules restrictions, the following methanol-blend alternatives are known to either increase power, improve efficiency, and/or suppress detonation and preignition .

Propylene Oxide: Ray Hall Turbo says adding 10 percent propylene oxide is worth about a 10 percent power increase. It's possible to run up to a 50/50 propylene oxide solution, but with anything over 10 percent, additional power gains aren't proportional to the added amount. To prevent corrosion as well as polymerization in the container that could cause a possible explosion, store propylene oxide in a polyethylene container in a cool location (the fuel boils at 93 degrees F), or polymerization in the container could result in an explosion.

Acetone: Up to 5 percent acetone can reduce preignition by raising the autoignition point. On a cold day, up to 10 percent acetone can ease initial start-up.

Legends persist of mixing hydrazine with nitromethane for a significant power gain. Hydrazine (N2H4) was developed in World War II as a rocket fuel. It powered the first operational rocket-based interceptor, the German Me-163B, and is still used in some spacecraft and myriad industrial processes to this day.

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A colorless, flammable liquid with an ammonia-like odor, hydrazine is so volatile that it's outlawed virtually everywhere. Jim Archer has some experience mixing nitro and hydrazine: "Yes you can do it, but it's dangerous as hell and very toxic." Nitro is slightly acidic, while hydrazine is slightly basic, and opposites attract, with a vengeance: When the two come into contact, a spontaneous chemical reaction starts that ultimately creates a salt-like, high-explosive compound that's extremely sensitive. So, any mix's efficacy for improving performance is extremely time sensitive. It takes a while for the reaction to really get going, so if you mix the two together and run it right away, nothing is gained. Somewhere around 25 to 30 minutes after mixing you will see a power gain over nitro alone. Around 45 minutes, the mix will blow up inside the engine or even self-detonate in the tank.

Nitromethane, dangerous as it is, looks like water compared with hydrazine. Don't breathe it, don't ingest it, and don't get it on your skin (it absorbs right through it).

Pass on the Gas

Nitro doesn't mix with gasoline-they separate, with the gas on top. You can, however, mix nitropropane (C3H7NO2) with gas, and even 10 percent nitropropane in gas can provide small power increases. A test by Jeff Smith is available at HOTROD.com/techarticles/42018/. Klotz (KlotzLube.com) sells a product called Nitro Power Additive, a mixture of nitropropane and antidetonation agents (nitropropane is extremely detonation sensitive).

What about running straight nitropropane? It provides about the same gain as running 60 percent nitromethane.

Tipping the Can And the Bottle

Nitrous oxide and nitromethane? It's possible-but only preliminary development has been done because most sanctioning bodies outlaw it. Mike Thermos says he's built systems that run up to 25 percent nitrous oxide, but even at that level, supplying enough fuel is problematic. You need huge fuel-side solenoids with special internal orifices capable of handling thick, viscous, lacquer-like nitromethane-probably at least two 0.180-inch-orifice models.

Applied Nitrous Technology specializes in hard-core systems for just about every race venue; as owner Jeff Prock puts it, "We've put nitrous on everything from model cars and lawn mowers up to Fuelers." More than a decade ago, Prock built a system for Keith Stark's A/Fuel Dragster. The goal was to prevent dropped cylinders induced by ignition that had trouble lighting off the 100 percent loads of nitromethane. It was hoped that nitrous would speed the flame front, which would permit retarding ignition timing, reducing ignition stress. Expectations were met: instead of 65 degrees, the engine ran best at 50 with nitrous. The nitrous jet sizes were roughly the size used on a 125hp system for gasoline car, with eight 0.018-inch orifice jets-but with nitro, that added more than 300 hp. Fuel flow at 6,300 rpm had been 31.1 gallons per minute; with the small nitrous shot that increased to 35.5 gpm. Times plummeted from 5.40 seconds to 5.29, and eventually 5.08. The big problem was shredding clutches.

Stark reports that the engine combo was never optimized for nitrous oxide: "We would have taken out some compression ratio if we kept pursuing it." Nitrous contains 36 percent oxygen by weight; nitromethane about 52 percent. High compression isn't needed with all that oxygen and fuel-you just want max volume of fuel into the cylinder. "You no longer have controlled combustion but a pure chemical reaction," warns Prock. Prock feels that with sufficient development, the normally aspirated, nitro/nitrous combination could have given blown Fuelers a run for their money. On the other hand, another nitromethane combo running 0.030-inch-orifice nitrous jets is said to have blown the side of the block clean off!

White-Hot

Several explanations are offered for the phenomenon of flames shooting out the exhaust pipes. Gene Adams says it's due to superchargers that "blow through unburned fuel on overlap." Flames have gotten longer as fuel pumps and magnetos have improved, allowing higher fuel volumes to be pushed through the engine. According to Adams, more volume means longer flames. "Back in the '60s and '70s, 1- to 2-foot flames were common. Now it's more like 10 feet."

An alternative explanation is that not all the nitro has the time to ignite within the engine and goes out the exhaust, where it ignites on contact with atmospheric oxygen, burning with a characteristic yellow flame. If a rich mixture has entered a monopropellant phase, hydrogen and carbon monoxide are produced as a byproduct. Bright white flames are then generated by burning hydrogen.

Hard to Start, Hard to Stop

Initial start-up with high nitro concentrations is very tricky. Jeff Prock says, "You must get the engine cycling. It won't start up spinning at 200 rpm like a gas engine would. You need to get some heat in the engine and spin it at 1,800 to 2,000 rpm." There's so much fuel pouring into the cylinders that failure to get the engine spinning fast enough before controlled ignition can hydro-lock the engine, or even blow a head off. The common practice is to start and warm up the engine on gas or alcohol.

High percentages of nitro required massive breakthroughs in ignition technology. Today's top-of-the-line MSD units put out 50,000 volts and 44 amps on the top end. That's about the output of an arc welder at each cylinder-and the Fuelers run two of them.

Once you get a nitro engine going, it may not want to stop. At 7,500 rpm on the top end, there's so much heat in the engine it may keep running under autoignition even if you shut off the magnetos. Essentially, it becomes a diesel. Fuelers today shut down by turning off the fuel pumps as well as the ignition.

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Don't Get Mixed Up

Correctly lending nitro involves far more than a mixing cup. The specific gravity (SG) of fuels varies per batch and purity; methanol's out-of-the-barrel concentration can vary 5 percent or so. The by-weight mixture is also dependent upon temperature, both during the initial mix as well as if the temperature changes afterward. As temperature rises, the weight (as measured by SG) decreases. For tuning, it's the percentage by weight that's important, so it is necessary to mix the fuel using a hydrometer and keep track of mixture temperature.

Stayin' Alive

Gene Adams, one of the old masters, offers some tidbits:

"Normally aspirated, nitromethane-fueled, engines don't like to rev as high as a gasoline- or alcohol motor. At high rpm, there's just too much volume of fuel and not enough time to burn it all. A blown alcohol dragster with a screw-compressor supercharger will run to 10,000 to 10,500 rpm; normally aspirated nitro cars in the same class run around 6,700 rpm and they make about the same power. If you try to rev them up higher, the power falls off. Adding a supercharger allows the engine to rev up higher, to about 8,200 on the top end.

"Lightweight parts are not as important since the engine isn't turning big rpm. Keeping everything together is, so we build things stouter-bigger piston wrist pins and heavier cranks, for example. Aluminum rods are obviously lighter than steel rods, but we use them for shock resistance, not weight savings. The 7075-T6 billet aluminum rods are much larger physically, so they are physically as strong as a mild-steel rod-but you can't keep bearings in a steel rod.

"Bearings, pistons, and everything have more clearance, and are run with heavy, single-weight 70W oil. This works better under nitro's tremendous loads. There's also a tremendous amount of blow-by even when running good, due to the tremendous internal pressure and rich fuel mixtures. There's a lot of cylinder-wall wash-down. Typically we use a Dykes top ring with a 0.017-inch-step, 116-inch second, and 316-inch heavy-tension oil ring.

"Nitro engines require lower compression ratios. Normally aspirated with modern race gas or methanol, 15:1 compression ratios are typical. With nitro, you're looking at about 10 to 11:1. A blown motor might run 6.0:1 compression with nitro, 8.5:1 with gasoline, and 12.0 to 13.0:1 on alcohol.

"Even with 'low' compression the Hemi heads are O-ringed, with the receiver groove in the block's sleeves. The ports are so huge you can put your legs in the heads. Valves sizes are 2.450-inch intake/1.94-inch exhaust.

"Typical cam specs might be 280-to-290-degrees duration at 0.050 with around 0.750-to-0.800-inch valve lift. That's not as radical as a Pro Stock car or alcohol dragster, but we don't turn the rpm, so why sacrifice durability? With so much ignition lead, excessive duration would only increase blow-through anyway. The cams are single-pattern because Hemis are not exhaust-side limited."

Running Out of Timing

Popularly, nitro is considered a "slow-burning" fuel, but the burn rate is between gas and methanol. The problem is that on high end, nitro-fueled engines, only about 10 percent of the fuel in the chamber is vapor when the burn starts; the rest is liquid. The vapor burns first, which ideally creates enough heat to vaporize the rest of the fuel. But it takes time to create that heat-hence the great amount of lead needed, about twice what you'd use with an equivalent gasoline-fueled engine. Adams says, "Normally aspirated we usually run 60 degrees of lead or 50 degrees with a supercharger. A gasoline-fueled, normally aspirated Hemi might only need 27 to 28 degrees."

Hammered

Nitromethane is weird stuff. You can strike a match next to a puddle of it and nothing will happen. But Jeff Prock says-based on personal experience when he was a kid-if you put a few drops of it on an anvil and hit it with a hammer, there will be a small explosion, somewhat akin to an old cap going off in a toy gun. More seriously, that means you don't want to risk dropping barrels off a truck. The explosion chance is remote, but it is possible, especially on a hot day.

16 HP/CI

With the current rules-restricted 90 percent nitro/10 percent methanol blend, modern Top Fuel 500ci Hemi engines make about 8,000 hp. That's 1,000 hp per cylinder or 16 hp/ci. It takes about 15/100 of a second for the power to reach the rear wheels.

Knock Your Block Off

If spark fails early in the run, unburned nitro can build up and then explode with a force that can blow the heads off the block-or even blow the block itself in half.